U.S. patent application number 12/385369 was filed with the patent office on 2009-10-15 for method for mode adaptation in mimo system.
Invention is credited to Jin Young CHUN, Bin Chul IHM, Hyun Soo KO, Moon Il LEE, Wook Bong LEE, Sung Ho PARK.
Application Number | 20090257384 12/385369 |
Document ID | / |
Family ID | 41162048 |
Filed Date | 2009-10-15 |
United States Patent
Application |
20090257384 |
Kind Code |
A1 |
LEE; Wook Bong ; et
al. |
October 15, 2009 |
METHOD FOR MODE ADAPTATION IN MIMO SYSTEM
Abstract
A mode adaptation method for performing mode switching between a
single-user MIMO (SU-MIMO) mode and a multi-user MIMO (MU-MIMO)
mode includes reporting, by a base station, a MIMO feedback mode to
a mobile station, receiving a feedback information determined on
the basis of the MIMO feedback mode from the mobile station; and
determining a MIMO feedback mode based on the feedback information.
The MIMO feedback mode indicates whether it is single-user MIMO or
multi-user MIMO and includes a number of mobile stations (MSs)
transferred at one time. As a result, the method maximizes a system
throughput simultaneously while effectively reducing an amount of
feedback information.
Inventors: |
LEE; Wook Bong; (Anyang-si,
KR) ; CHUN; Jin Young; (Anyang-si, KR) ; PARK;
Sung Ho; (Anyang-si, KR) ; LEE; Moon Il;
(Anyang-si, KR) ; KO; Hyun Soo; (Anyang-si,
KR) ; IHM; Bin Chul; (Anyang-si, KR) |
Correspondence
Address: |
MCKENNA LONG & ALDRIDGE LLP
1900 K STREET, NW
WASHINGTON
DC
20006
US
|
Family ID: |
41162048 |
Appl. No.: |
12/385369 |
Filed: |
April 6, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61042778 |
Apr 7, 2008 |
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61074132 |
Jun 19, 2008 |
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61050591 |
May 5, 2008 |
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61151161 |
Feb 9, 2009 |
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Current U.S.
Class: |
370/329 ;
375/267 |
Current CPC
Class: |
H04B 7/0639 20130101;
H04B 7/0689 20130101; H04B 7/0632 20130101; H04B 7/065 20130101;
H04B 7/0417 20130101; H04B 7/0452 20130101 |
Class at
Publication: |
370/329 ;
375/267 |
International
Class: |
H04W 72/04 20090101
H04W072/04; H04B 7/02 20060101 H04B007/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 20, 2008 |
KR |
10-2008-0081244 |
Claims
1. A mode adaptation method for performing mode switching between a
single-user MIMO (SU-MIMO) mode and a multi-user MIMO (MU-MIMO)
mode, the method comprising: reporting, by a base station, a MIMO
feedback mode to a mobile station; receiving a feedback information
determined on the basis of the MIMO feedback mode from the mobile
station; and determining a MIMO feedback mode based on the feedback
information, wherein the MIMO feedback mode indicates whether it is
single-user MIMO or multi-user MIMO and includes a number of mobile
stations (MSs) transferred at one time.
2. The mode adaptation method according to claim 1, wherein the
MIMO feedback mode is one of a single-user MIMO (SU-MIMO) mode, a
multi user MIMO (MU-MIMO) mode 2 supporting two mobile stations, a
MU-MIMO mode 3 supporting three mobile stations and a MU-MIMO mode
4 supporting four mobile stations.
3. The mode adaptation method according to claim 2, wherein the
feedback information includes at least one of CQI, PMI, and
Rank.
4. A mode adaptation method for performing mode switching between a
single-user MIMO (SU-MIMO) mode and a multi-user MIMO (MU-MIMO)
mode, the method comprising: receiving, by a mobile station, a MIMO
feedback mode from a base station; transmitting a feedback
information determined on the basis of the MIMO feedback mode to
the base station; and receiving a MIMO feedback mode determined
based on the feedback information from the base station; wherein
the MIMO feedback mode indicates whether it is single-user MIMO or
multi-user MIMO and includes a number of mobile stations (MSs)
transferred at one time.
5. The mode adaptation method according to claim 4, wherein the
MIMO feedback mode is one of a single-user MIMO (SU-MIMO) mode, a
multi user MIMO (MU-MIMO) mode 2 supporting two mobile stations, a
MU-MIMO mode 3 supporting three mobile stations and a MU-MIMO mode
4 supporting four mobile stations.
6. The mode adaptation method according to claim 5, wherein the
feedback information includes at least one of CQI, PMI, and Rank.
Description
[0001] This application claims the benefit of the Korean Patent
Application No. 10-2008-0081244, filed on Aug. 20, 2008, which is
hereby incorporated by reference as if fully set forth herein.
[0002] This application also claims the benefit of U.S. Provisional
Application Ser. Nos. 61/042,778, filed on Apr. 7, 2008,
61/074,132, filed on Jun. 19, 2008, 61/050,591, filed on May 5,
2008, and 61/151,161, filed on Feb. 9, 2009, the contents of which
are hereby incorporated by reference herein in their entirety.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to a mode adaptation method
for a Multi-Input Multi-Output (MIMO) system, and more particularly
to a method for reducing an amount of uplink feedback
information.
[0005] 2. Discussion of the Related Art
[0006] Closed-loop MIMO (CL-MIMO) systems, which are adaptively
operated according to a channel condition of a user, among all MIMO
systems based on a multi-antenna can greatly improve a performance
or throughput of an overall system.
[0007] The closed-loop MIMO systems can be classified into a
codebook based preceding system for reporting a preceding matrix
index (PMI) and an analog feedback based preceding system for
reporting a channel. The codebook based preceding system and the
analog feedback based preceding system may have the following
advantages and disadvantages.
[0008] The codebook based precoding system requires a small amount
of feedback information, has a small number of channel quality
indicator or information (CQI) mismatches, and reduces a flashlight
effect according to codeword restriction. At this time, the CQI may
be measured in either a midamble or common pilots on the basis of
complete comprehension of both a precoder and an interferer.
[0009] The codebook based precoding system has the following
disadvantages. In more detail, the codebook based precoding system
has a large number of quantization errors and a low beamforming
gain. In order to implement the above-mentioned codebook based
preceding system, an appropriate codebook design is needed.
[0010] The analog feedback based preceding system reduces the
number of quantization errors and acquires a high beamforming gain.
On the other hand, the analog feedback based preceding system
requires an excessive amount of either feedback information or
feedback errors, has a large number of CQI mismatches, and
generates the flashlight effect. Specifically, if the analog
feedback based preceding is used for a multi-user MIMO system, the
CQI can be measured in only dedicated pilots. Also, provided that
the analog feedback based preceding system does not recognize the
interfering precoder, the mobile station is unable to measure the
CQI. In order to implement the analog feedback based preceding
system, a sounding channel is needed.
[0011] Therefore, the codebook based preceding system has the
above-mentioned advantages, such that it has been widely used for a
3GPP LTE system, a WiMAX system, and a 2GPP2 Ultra Mobile Broadband
(UMB) system.
[0012] The MIMO systems are classified into a single user MIMO
(SU-MIMO) system and a multi-user MIMO (MU-MIMO) system according
to methods for allocating the spatial resources.
[0013] FIGS. 1 and 2 illustrate block diagrams illustrating
transmission structures when data is transferred at two or more
spatial multiplexing rates.
[0014] FIG. 1 shows a case in which a vertical encoding (or a
single codeword (SCW)) are used. FIG. 2 shows another case in which
horizontal encoding (or a multi-codeword (MCW)) are used.
[0015] The SU-MIMO system allocates all of the spatial resources to
only one mobile station (MS). When the SU-MIMO system is operated
under a closed-loop MIMO scheme, each mobile station (MS) selects a
preferred rank (i.e., a spatial multiplexing rate), and reports the
preferred rank, a preceding matrix index (PMI) and a channel
quality information (CQI) which are suitable for the selected rank.
A base station (BS) allocates only one mobile station (MS) to a
resource (i.e., time and frequency) using such feedback
information. In this case, the spatial resources are all used by
the mobile station (MS). Here, the SCW or MCW may be used as the
transmission structure.
[0016] The MU-MIMO system is used to allocate spatial resources to
a few mobile stations. When the MU-MIMO system is operated under
the CL-MIMO scheme, each mobile station (MS) transmits the CQI and
the PMI according to MU-MIMO conditions. In this case, the MU-MIMO
conditions are a preceding matrix set, MU-MIMO types (PU2RC, ZF-BF,
. . . ), and the like. The base station (BS) selects mobile
stations (MSs) satisfying a specific condition using the received
information, and allocates the selected mobile stations to a
resource (i.e., time and frequency). Here, the transmission
structure is set to the MCW.
[0017] Generally, if the number of users is small, the SU-MIMO
system has a good throughput superior to that of the MU-MIMO
system. If the number of users is large, the MU-MIMO system has a
good throughput superior to that of the SU-MIMO system. When
several users are paired with each other in the MU-MIMO system, if
orthogonal pairs of users are found, the throughput of the MU-MIMO
system becomes better. The larger the number of users, the higher
the probability of generating the orthogonal pairs.
[0018] FIG. 3 is a graph showing a comparison of throughput between
the SU-MIMO system and the MU-MIMO system.
[0019] In FIG. 3, if about 5 or more people are in a cell, the
MU-MIMO system has a good throughput superior to that of the
SU-MIMO system. This specific point is called an SU-MU switching
point. This switching point may occur at different positions
according to a channel status, a user status and the like.
[0020] In more detail, a throughput of an optimum system must
follow envelopes of the SU-MIMO throughput and the MU-MIMO
throughput. For this operation, the mobile station (MS) must carry
out not only a feedback operation suitable for the SU-MIMO system
but also another feedback operation suitable for the MU-MIMO
system. As a result, an amount of feedback overhead unavoidably
increases.
[0021] Generally, a codebook structure is pre-decided.
Specifically, in case of the MU-MIMO mode, if the number of users
is not very large, a codebook size must be small such that a
pairing gain arises. Therefore, the codebook size for the MU-MIMO
mode has generally been designed to be smaller than that of the
SU-MIMO mode, such that a beamforming gain decreases.
SUMMARY OF THE INVENTION
[0022] Accordingly, the present invention is directed to a mode
adaptation method for a MIMO system that substantially obviates one
or more problems due to limitations and disadvantages of the
related art.
[0023] An object of the present invention is to provide a mode
adaptation method for maximizing a system throughput simultaneously
while effectively reducing an amount of feedback information.
[0024] Additional advantages, objects, and features of the
invention will be set forth in part in the description which
follows and in part will become apparent to those having ordinary
skill in the art upon examination of the following or may be
learned from practice of the invention. The objectives and other
advantages of the invention may be realized and attained by the
structure particularly pointed out in the written description and
claims hereof as well as the appended drawings.
[0025] To achieve these objects and other advantages and in
accordance with the purpose of the invention, as embodied and
broadly described herein, there is provided a mode adaptation
method for performing mode switching between a single-user MIMO
(SU-MIMO) mode and a multi-user MIMO (MU-MIMO) mode includes
reporting, by a base station, a MIMO feedback mode to a mobile
station; receiving a feedback information determined on the basis
of the MIMO feedback mode from the mobile station; and determining
a MIMO feedback mode based on the feedback information. The MIMO
feedback mode indicates whether it is single-user MIMO or
multi-user MIMO and includes a number of mobile stations (MSs)
transferred at one time.
[0026] The MIMO feedback mode may be one of a single-user MIMO
(SU-MIMO) mode, a multi user MIMO (MU-MIMO) mode 2 supporting two
mobile stations, a MU-MIMO mode 3 supporting three mobile stations
and a MU-MIMO mode 4 supporting four mobile stations.
[0027] The feedback information may include at least one of CQI,
PMI, and Rank.
[0028] In another aspect of the present invention, there is
provided a mode adaptation method for performing mode switching
between a single-user MIMO (SU-MIMO) mode and a multi-user MIMO
(MU-MIMO) mode, the method including: receiving, by a mobile
station, a MIMO feedback mode from a base station; transmitting a
feedback information determined on the basis of the MIMO feedback
mode to the base station; and receiving a MIMO feedback mode
determined based on the feedback information from the base station.
The MIMO feedback mode indicates whether it is single-user MIMO or
multi-user MIMO and includes a number of mobile stations (MSs)
transferred at one time.
[0029] The MIMO feedback mode may be one of a single-user MIMO
(SU-MIMO) mode, a multi user MIMO (MU-MIMO) mode 2 supporting two
mobile stations, a MU-MIMO mode 3 supporting three mobile stations
and a MU-MIMO mode 4 supporting four mobile stations.
[0030] The feedback information may include at least one of CQI,
PMI, and Rank.
[0031] It is to be understood that both the foregoing general
description and the following detailed description of the present
invention are exemplary and explanatory and are intended to provide
further explanation of the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] The accompanying drawings, which are included to provide a
further understanding of the invention and are incorporated in and
constitute a part of this application, illustrate embodiment(s) of
the invention and together with the description serve to explain
the principle of the invention. In the drawings:
[0033] FIGS. 1 and 2 illustrate block diagrams illustrating
transmission structures when data is transferred at two or more
spatial multiplexing rates;
[0034] FIG. 3 is a graph showing a comparison of throughput between
an SU-MIMO and an MU-MIMO;
[0035] FIG. 4 is a flow chart illustrating a method for reducing an
amount of feedback information using a mode adaptation method
according to one embodiment of the present invention;
[0036] FIG. 5 shows a method for transferring information of a
preferred mode, instead of a CQI and PMI of a current mode, over a
feedback channel at intervals of a specific period;
[0037] FIG. 6 shows a method for periodically transferring
information of a preferred mode over an additional feedback
channel;
[0038] FIG. 7 shows an exemplary case for transferring rank
information instead of preferred mode information;
[0039] FIG. 8 shows a method for requesting a feedback of a
specific mode by a base station (BS) and performing the feedback of
the requested mode by a mobile station (MS); and
[0040] FIG. 9 is a graph illustrating a throughput of a mode
adaptation method according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0041] Reference will now be made in detail to the preferred
embodiments of the present invention, examples of which are
illustrated in the accompanying drawings. Wherever possible, the
same reference numbers will be used throughout the drawings to
refer to the same or like parts. The following embodiments of the
present invention may be modified into various formats, and the
scope of the present invention is not limited to only the following
embodiments and can also be applied to other examples.
[0042] FIG. 4 is a flow chart illustrating a method for reducing an
amount of feedback information using a mode adaptation method
according to the present invention.
[0043] In FIG. 4, l is the number of mobile stations (MSs)
transferred at one time under a specific mode (l), and M is the
number of mobile stations (MSs) transferred at one time under
another mode (M).
[0044] The mode may be indicated from the base station (BS). The
mode may be set to the mode M which is transferred through a
downlink control channel such as an IEEE 802.16e's MAP or a
submap.
[0045] .gamma..sub.l may be calculated by the mobile station, and
may be a capacity or throughput as a metric of the l-th mode. In
this case, it is assumed that l*.gamma..sub.l and M*.gamma..sub.M
are called throughput metrics. C.sub.1 is a metric calculated by
the base station on the basis of a feedback of the mobile station.
For example, C.sub.l may be a sum average capacity (SAC) or a sum
average throughput (SAT), etc.
[0046] .alpha. is a threshold value for a feedback. If a specific
condition
l .gamma. l M .gamma. M > .alpha. ##EQU00001##
is provided, the mobile station reporting the l-th mode's feedback.
.alpha. may be a value indicated by the base station, a
predetermined value, or another value arbitrarily decided by each
mobile station.
[0047] In FIG. 4, the mobile station determines whether a ratio of
a throughput metric of a current mode (M) to a throughput metric of
another mode (l) to which a MIMO mode will be switched is higher
than a threshold value at step 410. In this case, if the above
ratio is higher than the threshold value, the mobile station
reports information of the mode (l) to which the MIMO mode will be
switched, to the base station.
[0048] Next, the base station having received feedback information
calculates the sum average throughput metric of the mode (l) using
feedback information at step 420, and determines whether the
calculated sum average throughput metric is higher than a mode
switching threshold value at step 430. FIG. 4 exemplarily shows a
sum average throughput metric of a current mode (M) as the mode
switching threshold value.
[0049] If the sum average threshold metric of the mode (l) is
higher than the mode switching threshold value at step 430, the
MIMO mode is changed to the mode (l), and the changed MIMO mode,
i.e., the mode (l) is notified to the mobile station at step 440.
At this time, if the threshold value a is changed, information of
the changed threshold value is also notified to the mobile
station.
[0050] In the meantime, if the sum average throughput metric of the
mode (l) is equal to or less than the mode switching threshold
value, only information of the threshold value .alpha. may be
notified to the mobile station at step 450.
[0051] Each mobile station may report mode information, a mode
index, a CQI and/or a PMI and the like to the base station in
various ways.
[0052] FIG. 5 shows a method for transferring information of a
preferred mode, instead of a CQI and PMI of a current mode, through
a feedback channel at intervals of a specific period through a
feedback channel.
[0053] Particularly, if an amount of information of the preferred
mode is larger than a capacity of a feedback channel, this
preferred mode information may be divisionally transferred several
times through the feedback channel. For some examples, the
following cases will be described in detail.
[0054] Provided that mode indication information is transferred at
intervals of four periods, the mobile station transmits feedback
information (CQI and/or PMI) of the mode M indicating a current
mode at the time indexes 0, 1 and 2 the feedback channel. If
another mode (mode l) does not satisfy
l .gamma. l M .gamma. M > .alpha. ##EQU00002##
at the time index 3 of the feedback channel, the mode M is fed back
as a preferred mode by the mobile station, and feedback information
(CQI and/or PMI) of the mode M being the current mode is then
transferred from the mobile station to the base station. If the
mode l satisfies
l .gamma. l M .gamma. M > .alpha. ##EQU00003##
at the time index 7 of the feedback channel indicating a mode
feedback period, feedback information (CQI and/or PMI) of the mode
l is transferred to the base station at the time index 8 of the
feedback channel. Thereafter, feedback information (CQI and/or PMI)
of the mode M indicating the current mode is transferred to the
base station. If the current mode M is changed to the mode l at the
time index 10 of the feedback channel, the mobile station transmits
feedback information (CQI and/or PMI) of the mode l to the base
station.
[0055] FIG. 6 shows a method for periodically transferring
information of a preferred mode through an additional feedback
channel.
[0056] Referring to FIG. 6, MS reports the CQI and/or PMI of the
current mode (mode M) via feedback channel 1, and MS reports
feedback information of another mode via feedback channel 2. For
example, if another mode (mode l) does not satisfy
l .gamma. l M .gamma. M > .alpha. ##EQU00004##
at a time index 0 of the feedback channel 2 (denoted by "feedback
channel (2) time index 0" in FIG. 6) so that the current mode is
considered to be an optimum mode, the mobile station reports the
mode M as a preferred mode to the base station. If the mode l
satisfies
l .gamma. l M .gamma. M > .alpha. ##EQU00005##
at a time index 1 of the feedback channel 2 (denoted by "feedback
channel (2) time index 1"), the mobile station reports that the
mode l is preferred, and then reports the CQI and/or PMI of the
mode l at a time index 2 of the feedback channel 2 (denoted by
"feedback channel (2) time index 2"). In this case, the mobile
station may report a preferred mode instead of reporting the CQI
and/or PMI of the mode l at the time index 2 of the feedback
channel 2, and may also transmit the CQI and/or PMI of the mode l
through another channel.
[0057] If another mode satisfies
l .gamma. l M .gamma. M > .alpha. ##EQU00006##
according to an event trigger scheme, the following two methods may
be used.
[0058] According to a first method, a mobile station requests an
additional feedback channel from a base station, and transmits the
CQI and/or PMI of the mode l through a feedback channel allocated
by the base station. According to a second method, if a mobile
station has data to be transferred to an uplink, it attaches the
CQI and/or PMI of the mode (l) to the channel of the data, and
transmits the attached result to the uplink.
[0059] FIG. 7 shows an exemplary case for transferring rank
information instead of preferred mode information.
[0060] If a current mode is set to the SU-MIMO mode, the mobile
station may transmit rank information instead of the CQI and/or PMI
of the mode M (SU-MIMO).
[0061] If the SU-MIMO rank adaptation period (i.e., a period for
selecting a preferred rank by the mobile station) is set to 4, the
mobile station transfers its own preferred rank at intervals of
four time indexes of feedback channel (1). At this time, if the
preferred rank is changed to another rank e.g., rank 2 in FIG. 7,
the mobile station transmits the CQI and/or PMI suitable for the
changed rank via additional feedback channel, e.g., feedback
channel (2). Meanwhile, the CQI and/or PMI suitable for the changed
rank may be transmitted via the feedback channel (1) through which
the information of the changed rank is transmitted at a period when
the information of the changed rank is transmitted without
allocating an additional feedback channel for the CQI and/or PMI.
In this case, the base station may allocate an additional feedback
channel for the rank information.
[0062] FIG. 8 shows a method for requesting a feedback of a
specific mode by a base station (BS) and performing the feedback of
the requested mode by a mobile station (MS).
[0063] Referring to FIG. 8, the base station may request a feedback
of the mode (l) from the mobile station. In other words, under the
condition that the current mode is the mode (M) and some mobile
stations perform a feedback of information of the mode (l), if the
base station requires more information of the mode (l), it may ask
the mobile station to feed back the information of the mode (l). In
this case, a feedback may be performed as follows. In more detail,
the mobile station reports information of a corresponding mode only
once or a predetermined number of times requested by the base
station, and then reports information of a current mode.
Specifically, if an amount of information of the requested mode is
larger than a capacity of a current feedback channel, this
requested mode information may be divisionally transferred several
times through the feedback channel. In the meantime, if the base
station asks the mobile station to perform a feedback of
information of another mode using the event trigger scheme, the
following methods may be used. According to a first method, the
base station allocates a new feedback channel, and transmits the
CQI and/or PMI of the requested mode through this newly-allocated
feedback channel (i.e., an additional feedback channel) According
to a second method, if the mobile station has data to be
transferred to an uplink, it attaches the CQI and/or PMI of the
requested mode to a channel of the data, and transmits the attached
result to the uplink.
[0064] As described above, if a user is located at a cell edge
(hereinafter referred to as a cell-edge user), the user may be
operated using the SU-MIMO scheme or another MIMO scheme (e.g., a
collaborative MIMO scheme) regardless of a mode of the cell. In
this case, the feedback may also be adjusted to the corresponding
scheme. Specifically, in case of using a fractional frequency reuse
(FFR) method, the FFR method allocates a specific frequency band to
a cell edge, reports only the specific frequency band (the
corresponding FFR band) according to the SU-MIMO scheme or the
collaborative MIMO scheme, and may also report the remaining bands
using a mode applied to a current cell in the same manner as in the
above-mentioned description.
[0065] In the meantime, some codebook elements are extracted from
the SU-MIMO codebook set on the condition that a codebook is
limited, such that the MU-MIMO codebook may be constructed.
[0066] In association with the MU-MIMO codebook, it is assumed that
the number of transmission (Tx) antennas is set to N.sub.t.
[0067] At this time, the mode may include the SU-MIMO mode, an
MU-MIMO mode having a rank 2, an MU-MIMO mode having a rank 3, . .
. , an MU-MIMO mode having a rank N.sub.t, etc. A precoding matrix
set of each mode can be represented by the following equation
1.
[0068] SU-MIMO
rank1: S.sub.1={{v.sub.1,1,1}, {v.sub.1,2,1} . . . ,
{v.sub.1,K1,1}}
rank2: S.sub.2={{v.sub.2,1,1, v.sub.2,1,2}, {v.sub.2,2,1,
v.sub.2,2,2} . . . , {v.sub.2,K2,1, v.sub.2,K2,2}}
. . .
rank Nt: S.sub.Nt={{v.sub.Nt,1,1, v.sub.Nt,1,Nt,}, {v.sub.Nt,2,1, .
. . , v.sub.Nt,2,Nt} . . . , {v.sub.Nt,K,1, . . . ,
v.sub.Nt,KNt,Nt}}
MU-MIMO with rank2: M.sub.2={{p.sub.2,1,1, p.sub.2,1,2},
{p.sub.2,2,1, p.sub.2,2,2} . . . , {p.sub.2,K2',1,
p.sub.2,K2',2}}
. . .
MU-MIMO with rankNt: M.sub.Nt={{p.sub.Nt,1,1, . . . ,
p.sub.Nt,1,Nt,}, {p.sub.Nt,2,1, . . . , p.sub.Nt,2,Nt} . . . ,
{p.sub.Nt,K,1, . . . , p.sub.Nt,KNt',Nt}} [Equation 1]
[0069] In Equation 1, K# is a codebook size of the SU-MIMO rank #,
and K#' is a codebook size of the MU-MIMO rank #. K# and K#' may be
different from each other as necessary.
[0070] In this case, the base station may inform the mobile station
of the K# and K#' values and constituent entities. If the base
station informs the mobile station of only the K# and K#' values,
it may use a specific codebook set predetermined for the K# or K#'
value.
[0071] A codebook of the feedback MU-MIMO can be constructed as
follows according to the following two MU-MIMO transmission
methods.
[0072] The MU-MIMO codebook performing a feedback using the
following two methods according to MU-MIMO transmission methods,
may be configured.
[0073] According to a first method, if a codebook being reporting
is different from a codebook transferred from the base station, the
MU-MIMO codebook for a feedback toward an uplink may include
codebook elements of a rank 1. The base station may use and combine
feedback information, such that it may configure another-format
precoder. In this case, the rank 1 codebook elements may be a
subset of the SU-MIMO rank (1) codebook.
[0074] According to a second method, if a feedback codebook is
equal to another codebook transferred from the base station, the
MU-MIMO codebook for a feedback toward the uplink can be operated
as follows. In the MU-MIMO of the rank 2 (denoted by "MU-MIMO with
rank 2"), a codebook includes codebook elements of the rank 2. In
the MU-MIMO of the rank 3 (denoted by "MU-MIMO with rank 3"), a
codebook includes codebook elements of the rank 3.
[0075] In this case, a preceding matrix set for each mode can be
represented by the following equation 2.
[0076] SU-MIMO
rank1: S.sub.1={{v.sub.1,1,1}, {v.sub.1,2,1} . . . ,
{v.sub.1,K1,1}}
rank2: S.sub.2={{v.sub.2,1,1, v.sub.2,1,2}, {V.sub.2,2,1,
v.sub.2,2,2} . . . , {v.sub.2,K2,1, v.sub.2,K2,2}}
. . .
rank Nt: S.sub.Nt={{v.sub.Nt,1,1, . . . , v.sub.Nt,1,Nt},
{v.sub.Nt,2,1, . . . , v.sub.Nt,2,Nt} . . . {V.sub.Nt,K,1, . . . ,
v.sub.Nt,KNt,Nt}}
MU-MIMO with rank2: M.sub.2={{v.sub.2,k1,1, v.sub.2,k1,2},
{v.sub.2,k2,1, v.sub.2,k2,2} . . . }
. . .
MU-MIMO with rankNt: MNt={{v.sub.Nt,k1,1, . . . , v.sub.Nt,k1,Nt},
v.sub.Nt,k2,1, . . . , v.sub.Nt,k2,Nt} . . . } [Equation 2]
[0077] In other words, the MU-MIMO codebook may extract some
constituent elements from the SU-MIMO codebook of the same
rank.
[0078] The base station changes the number of codebook elements
constructing the MU-MIMO and their constituent entities to others,
and may inform the mobile station of both the changed number of
codebook elements and the changed constituent entities. For
example, in case of a codebook entry, the base station may extract
the codebook entry on the basis of SU-MIMO feedback information
received from the mobile station, and may broadcast the constructed
codebook set to the mobile station. In this case, K#' may be
predetermined for each rank, or may be indicated to the mobile
station. The constructed codebook includes some parts of preferred
codebook indexes of each mobile station. So, although the MU-MIMO
system uses a small-sized codebook set, it can acquire a higher
beamforming gain.
[0079] Specific constituent elements of the SU-MIMO codebook may
not be used due to a request from another cell or other reasons.
For example, if another cell mobile station reports that a specific
codebook element (e.g., v.sub.1,k,1) influences greatly due to
interference, this constituent element may not be used. In
addition, if .alpha.v.sub.1,k,1 is contained in a codebook of a
higher rank, this codebook element may not be also used.
[0080] The MU-MIMO operations based on the above-mentioned codebook
are as follows.
[0081] First, if the rank of the MU-MIMO codebook is equal to a
transmission rank, i.e., if the MU-MIMO codebook of the rank 2 is
configured in the form of Nt.times.2 matrix, the mobile station
calculates the CQI using a reception algorithm for each codebook
element, and feeds back its own preferred precoding matrix index
(PMI), a stream index, a CQI, etc. The base station performs
pairing of mobile stations, and applies a corresponding codebook to
the paired mobile stations.
[0082] Next, if the codebook rank of the MU-MIMO for a feedback
toward an uplink is set to 1, i.e., if the MU-MIMO codebook is
configured in the form of an Nt.times.1 matrix, the mobile station
calculates the CQI on the assumption of a single rank transmission
(rank1 transmission) for each codebook element, and reports a
precoding matrix/vector index, and the CQI. Otherwise, provided
that orthogonal or almost-orthogonal codebook elements of
individual codebook elements are paired with each other, the mobile
station calculates the CQI using a reception algorithm implemented
by the MU-MIMO mode. The preceding matrix/vector index and the CQI
are reported from the mobile station.
[0083] The base station performs pairing of mobile stations using
such feedback information, modifies the CQI as necessary, and then
selects an MCS level. The base station modifies a precoding matrix
according to a selected MU-MIMO algorithm (e.g., ZF-BF, PU2RC, or
SDMA), and transmits the modified preceding matrix to each mobile
station.
[0084] FIG. 9 is a graph illustrating a throughput of a mode
adaptation method (SU-MIMO or MU-MIMO) according to the present
invention.
[0085] As can be seen from FIG. 9, it can be readily recognized
that a throughput graph of the mode adaptation method according to
one embodiment of the present invention closely follows envelopes
indicating optimum throughputs of the SU-MIMO and the MU-MIMO.
[0086] Although the present invention has been disclosed by
referring to the above-mentioned embodiments, it should be noted
that the aforementioned embodiments have been disclosed for only
illustrative purposes, and those skilled in the art will appreciate
that various modifications, additions and substitutions are
possible, without departing from the scope and spirit of the
invention as disclosed in the accompanying claims. Thus, it is
intended that the present invention covers the modifications and
variations of this invention provided they come within the scope of
the appended claims and their equivalents.
[0087] As apparent from the above description, the present
invention provides a method for reducing an amount of uplink
feedback information using a mode adaptation method of the MIMO
system. This method can be applied to a base station, a mobile
station and the like of the IEEE 802.16m system. So, the present
invention can maximize a system throughput simultaneously while
effectively reducing an amount of feedback information.
* * * * *